feat(skills): add coding-dev.md and per-language rules under code-languages/
Generic coding rules and preferences distilled from existing agent rule files, project AGENTS.md/.rules, and saved memories: - strictness maximal; suppressions must be reported to the user - never filter build/lint/test output through grep/tail/head - reuse > fork > self-implement; generic code becomes its own package - 1000-line file / 100-line function limits, split by concern - regression test every fixed bug; never test against real user data - events over polling; scope discipline; honest reporting Per-language: rust, js-typescript, bash, python, go, csharp-dotnet, c-cpp. Each extends coding-dev.md rather than repeating it. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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---
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name: c-cpp
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description: >-
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Use this skill when writing, building, or debugging C or C++ — including compiler warning
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configuration, sanitizers, modern C++ resource management, build systems (CMake/Meson),
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32-bit/cross builds, and FFI boundaries exposed to other languages.
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---
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# C / C++ — Rules & Preferences
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Extends [`../coding-dev.md`](../coding-dev.md). Everything there applies; this adds C/C++ specifics.
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C and C++ are for the cases with no better option: existing codebases, hard real-time paths,
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and FFI boundaries. **For new standalone work, prefer Rust** — the memory-safety guarantees
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are worth more than the familiarity.
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---
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## 1. Warnings — Everything On, Errors
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```cmake
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target_compile_options(mytarget PRIVATE
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-Wall -Wextra -Wpedantic -Werror
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-Wshadow -Wconversion -Wsign-conversion
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-Wnon-virtual-dtor -Wold-style-cast -Wcast-align
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-Wunused -Woverloaded-virtual -Wdouble-promotion
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-Wformat=2 -Wnull-dereference -Wimplicit-fallthrough
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)
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```
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- **`-Werror` in the build gate.** `-Wconversion` and `-Wshadow` are noisy on first contact and
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catch real bugs — do not drop them because a legacy file complains; fix the file or scope the
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suppression to it.
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- Run **both** GCC and Clang where possible; each diagnoses things the other misses.
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- `clang-tidy` with `bugprone-*`, `cert-*`, `performance-*`, `modernize-*`.
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- `clang-format` with a committed `.clang-format`. No hand-formatting.
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---
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## 2. Sanitizers — Use Them
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```bash
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cmake -DCMAKE_BUILD_TYPE=Debug \
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-DCMAKE_CXX_FLAGS="-fsanitize=address,undefined -fno-omit-frame-pointer"
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```
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- **ASan + UBSan on every debug/test build.** They find the bugs that warnings cannot.
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- TSan for anything threaded (separate build — it is incompatible with ASan).
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- Valgrind when a sanitizer build is not possible.
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- **A sanitizer finding is a bug, not noise.** Fix it.
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---
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## 3. Resource Management (C++)
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- **RAII for everything.** Every raw `new`/`delete`, `malloc`/`free`, `fopen`/`fclose`, and
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native handle pair is a leak waiting for an early return or a throw.
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- `std::unique_ptr` by default; `std::shared_ptr` only where ownership is genuinely shared.
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`std::weak_ptr` to break cycles.
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- **No owning raw pointers.** A raw pointer or reference in a signature means "borrowed,
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non-owning, outlives the call".
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- `std::string_view` / `std::span` for non-owning views — and never return one that outlives
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its backing storage.
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- **The Rule of Zero**: design so no destructor, copy, or move needs writing. If you must write
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one, write all five.
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- `constexpr`, `const`, and `noexcept` wherever they hold. `[[nodiscard]]` on anything whose
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return value must not be dropped.
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---
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## 4. C Specifics
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- C17 or later unless the target forbids it.
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- **Check every allocation and every return value.** In C, unchecked returns are the primary
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bug source.
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- Fixed-width types from `<stdint.h>` (`uint32_t`), never bare `int` for anything with a
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defined width on the wire.
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- Always the bounded string functions (`snprintf`, `strncpy` with explicit termination); never
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`strcpy`, `sprintf`, `gets`.
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- `static` on every function and global not exported from the translation unit.
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- Prefer a struct-and-functions module boundary with an opaque pointer over exposing layout.
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---
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## 5. Build Systems
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- **CMake** (3.20+, target-based) or **Meson** for new projects. Never a hand-written Makefile
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for anything non-trivial.
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- Target-based CMake only: `target_link_libraries`, `target_include_directories`,
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`target_compile_options`. **No directory-level `include_directories` or global
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`CMAKE_CXX_FLAGS` mutation.**
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- `PRIVATE` / `PUBLIC` / `INTERFACE` chosen deliberately — this is what keeps include leaks out
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of dependents.
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- **Register every new source file with the build system** in the same commit that adds it. A
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source file the build never compiles will sit there looking correct while every edit to it
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does nothing.
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- Vendor dependencies with `FetchContent` or submodules; document which and why.
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---
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## 6. Cross & Multi-Arch Builds
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- Keep 32-bit and 64-bit outputs in **separate build directories**; never reuse one.
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- **A component built for one architecture cannot load one built for another.** An
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`ELFCLASS64` / `ELFCLASS32` error means a loader path is reaching the wrong build — check the
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library search path, not the code.
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- When a build must match an already-installed runtime's version, **pin it explicitly** and
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note the constraint. A silently mismatched pair fails at load time with a message that
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points nowhere useful.
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- Record the exact toolchain and container used for a cross build. "It built on my machine" is
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not reproducible for native code.
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---
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## 7. FFI Boundaries
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When exposing C/C++ to another language:
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- `extern "C"` on the boundary, with a hand-written, stable, documented header.
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- **Ownership across the boundary must be explicit** — who allocates, who frees, and with which
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function. Provide a matching `free` for every allocation you hand out.
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- **Never let an exception cross an `extern "C"` boundary.** Catch everything at the edge and
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convert to an error code.
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- Bindings generated by a tool (`bindgen`, etc.) are regenerated, never hand-edited.
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- On the consuming side, keep all `unsafe`/unmanaged interop in **one thin wrapper module**
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that exposes a safe API — never scattered through call sites.
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> [!WARNING]
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> **Binding crates/packages that reach into a library's struct fields break when that library
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> makes the struct opaque.** Pin to a binding version that matches the installed library
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> generation, and note the constraint where someone will read it before bumping.
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---
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## 8. Testing & Debugging
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- GoogleTest or Catch2; CTest integration so `ctest` runs the suite.
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- **Regression test every fixed bug.**
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- Debug builds keep frame pointers (`-fno-omit-frame-pointer`) so stack traces are usable.
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- `-g` even in release builds; ship separate debug symbols rather than none.
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- **Record dead ends in the project's notes**, especially for native debugging — the
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investigation that ruled something out is expensive and will otherwise be repeated. Note
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what did *not* work alongside what did.
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- When a binary search through a packed/embedded blob finds nothing, that is not evidence the
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content is absent — packed asset formats defeat naive string searches. Re-check without
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anchoring before concluding.
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---
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## 9. Related
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- [`../coding-dev.md`](../coding-dev.md) — the general rules this extends.
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- [`rust.md`](./rust.md) — the preferred language for new native work, and the usual consumer
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of these FFI boundaries.
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